Bison Parasites are Betting on Scheduled Deworming

A new Texas A&M study of bison producers finds most are deworming on a schedule rather than a diagnosis, and researchers say that habit could be quietly breeding drug-resistant parasites.

Gastrointestinal parasites are a serious, costly problem in North American bison, but the drugs producers use to control them can lose their punch fast when they're used too often, or used without knowing which animals actually need treatment. A new study out of the Texas A&M College of Veterinary Medicine and Biomedical Sciences set out to find out how bison producers are actually handling parasite control on the ground, and the answers point toward a habit that could be working against them.

“Gastrointestinal nematodes are one of the most economically important health concerns for bison,” said Dr. Kaylee Kipp, a recent VMBS doctoral graduate now teaching ranch management at Texas Christian University. Bison, she said, seem to carry a heavier parasite burden than beef cattle, based on both anecdotal reports and diagnostic evidence.

Left unmanaged, these worms cause diarrhea, poor weight gain, and reduced production, and in severe cases can contribute to the death of adult bison. That makes control genuinely important. It just doesn't mean deworming more often is the fix.

Surprisingly little has been documented about how ranched bison are actually managed, especially in the southern U.S. Most existing bison research comes out of larger herds in northern states and Canada, leaving a real gap around operations in Texas and neighboring states.

“We needed to understand the basics of these operations,” said Dr. Guilherme Verocai, an associate professor in VMBS' Department of Veterinary Pathobiology and director of its Parasitology Diagnostic Laboratory. “Are we talking about a herd of 500 animals or 30? Those can be very different operations.”

To find out, the research team surveyed bison producers across herd demographics, pasture management, nutrition, animal sourcing, and parasite control practices. Nearly 90% said they dewormed their bison at least once a year, and almost a third dewormed more than twice annually. Diagnostic tools that could actually confirm whether treatment was needed, meanwhile, were rarely used. For the most part, producers were leaning on practices borrowed directly from the cattle industry.

Bison complicate that borrowed playbook. They're harder and more dangerous to handle than cattle, which makes frequent trips through a handling facility a real obstacle, pushing some producers toward alternatives like medicated feed instead. And the drug options themselves are thinner on the ground: only one product is FDA-approved specifically to treat gastrointestinal nematodes in bison, which means most treatment decisions still borrow doses and strategies developed for cattle or other ruminants, species researchers don't yet know are even the right model for bison.

The bigger concern sitting underneath all of this is anthelmintic resistance, worms evolving to survive the drugs meant to kill them. The mechanism mirrors antibiotic resistance: treatment kills off susceptible parasites while resistant ones survive and reproduce, and repeated treatment gradually tips the population further toward resistance.

“The concern is that over time, we could reach a point where the majority of those parasites are resistant,” Verocai said, noting that with few dewormer drug classes available and few new ones on the horizon, treatment has to be one tool among several rather than the whole strategy.

One of those other tools is diagnostic testing. A fecal test can show which parasites are actually present and whether treatment is warranted at all, letting producers and veterinarians make targeted calls instead of blanket ones.

“What might look like extra labor, collecting the fecal sample, sending it to the lab, and waiting on those results, most likely means you would treat fewer animals and save money on drug use,” Verocai said. Treating fewer animals also means less selective pressure pushing parasite populations toward resistance, which helps keep the limited drugs on hand effective for longer.

A second strategy, called refugia, works almost counterintuitively: deliberately leaving some animals untreated so drug-susceptible parasites stick around in the population and keep breeding with resistant ones, diluting resistance rather than accelerating it. Fewer than 10% of surveyed producers said they intentionally leave animals untreated for that purpose.

“Refugia is a huge topic and needs to be talked about more,” Kipp said. “We can't only be thinking about the next growing season, the next breeding season, or the next calving season. We have to be thinking about five, 10, or 20 years down the road.”

None of this is an argument for treating parasites less seriously. It's an argument for treating them more deliberately, based on what a given herd actually needs rather than a fixed calendar.

“Every time you buy an animal, you are buying parasites,” Kipp said. “You are essentially not only a bison manager, but you're also a parasite manager in a way, because they're not going to go away.”

Because herd size, environment, and management all vary so widely, the researchers aren't chasing a single one-size-fits-all protocol. The survey is meant to establish a baseline, one the team hopes will eventually support bison-specific recommendations that protect both animal health and the small handful of drugs producers have left to work with.

“It's a double-edged sword,” Kipp said. “Parasites can cause serious problems in bison, so the natural response is to deworm them, but more treatment isn't always better. We need to use these drugs strategically and be good stewards of them because we only have so many options.”

Source: Texas A&M College of Veterinary Medicine and Biomedical Sciences, featuring research from Dr. Kaylee Kipp (VMBS/Texas Christian University) and Dr. Guilherme Verocai (VMBS Department of Veterinary Pathobiology).